issue89: vagrant status is now displaying task_state if not null and oth...

...erwise vm_state

Now a clean pull request ( I hope )

NB: #issue91 ( already coded except tests unit) depend on that PR
I would just have reversed the condition to avoid the use of the negative conditition

 ```ruby
if server['OS-EXT-STS:task_state'].nil?
  server['status'].downcase.to_sym
else 
  server['OS-EXT-STS:task_state'].downcase.to_sym
end
```

If you can do this in a new PR, that would be great. Thanks